Various types of computing, entertainment, and/or mobile devices can be implemented with a transparent or semi-transparent display through which a user of a device can view the surrounding environment. Such devices, which can be referred to as see-through, mixed reality display device systems, or as augmented reality (AR) systems, enable a user to see through the transparent or semi-transparent display of a device to view the surrounding environment, and also see images of virtual objects (e.g., text, graphics, video, etc.) that are generated for display to appear as a part of, and/or overlaid upon, the surrounding environment. These devices, which can be implemented as head-mounted display (HMD) glasses or other wearable display devices, but are not limited thereto, often utilize optical waveguides to replicate an image, e.g., produced by a display engine, to a location where a user of a device can view the image as a virtual image in an augmented reality environment. As this is still an emerging technology, there are certain challenges associated with utilizing optical waveguides to display images of virtual objects to a user.
In HMDs and other types of imaging devices that utilize optical waveguides, such as heads up displays (HUDs), light propagates through the optical waveguide only over a limited range of internal angles. Light propagating parallel to the surface will, by definition, travel along the waveguide without bouncing. Light not propagating parallel to the surface will travel along the waveguide bouncing back and forth between the surfaces, so long as the angle of incidence with respect to the surface normal is greater than some critical angle associated with the material from which the optical waveguide is made. For example, for BK-7 glass, this critical angle is about 42 degrees. This critical can be lowered slightly by using a reflective coating, or by using a material having a higher index of refraction, which is typically more expensive. Regardless, the range of internal angles over which light will propagate through an optical waveguide does not vary very much, and for glass, the maximum range of internal angles is typically below 50 degrees. This typically results in a range of angles exiting the waveguide (i.e., angles in air) of less than 40 degrees, and typically even less when other design factors are taken into account.